docs: def get_runner(worker_type, max_workers=None, workers_window=None):
      """returns a runner callable.
  
      :param str worker_type: one of `simple` or `thread`.
      :param int max_workers: max workers the runner can spawn in parallel.
      :param in workers_window: max number of jobs waiting to be done by the
        workers at any given time.
      :return:
      """
      worker_func = _runners_mapping[worker_type]
      return partial(
          worker_func, max_workers=max_workers, workers_window=workers_window
      )